Noise limiting circuit



' diode conduct/on.

March 24, 1942, I R DO E -'2,277,040

NOISE LIMITING CIRCUIT Filed May 18, 1940 Fig 2.

Volta ge on diode load resistor Plate vo/ta e on audio amp ie r' in absence of llmlten' Fig. 4. Voltage effecting 40 Fig. 5. E

Inventor: Robert B.Dom e, 9 iv 6.

His Attorney.

Patented Mar. 24, 1942 NOISE LIIVHTING CIRCUIT Robert B. Dome, Bridgeport, (101111., assignor to General Electric Company, a corporation of New York Application May 18, 1940, Serial No. 335,975

2 Claims.

My invention relates to a noise limiting circuit and particularly to such a circuit suitable for use as a component element of a radio receiving system.

One of the main objects of my invention is to provide an improved noise limiting circuit which is effective to separate sudden interfering noise impulses from desired signals and substantially to prevent such impulses from appearing in the signal output circuits of the receiver.

Another object of my invention is to provide a very simple, economical and effective noise limiter which is readily adapted to existing receiver circuits.

In accordance with my invention a network including a unilaterally conducting discharge path is provided in shunt to a signal source, the path functioning automatically to maintain itself biased to a substantially nonconducting state in accordance with the envelope of the desired signals and becoming conductive to provide a low impedance by-pass circuit for noise impulses exceeding the signal envelope.

More specifically it is an object of my invention to provide an improved noise limiting circuit for the audio frequency circuits of a radio receiving system, or the video circuits of a television receiver, which automatically maintains a limiting level for noise impulses substantially following the signalfrequency envelope.

A further object of my invention is to provide an improved audio frequency noise limiter for limiting noise impulses to a level which is capable of varying automatically with audio fre- .quency variations at a rate approximating the syllable inflections of the human voice.

The features of my invention which I believe to be novel are set forth with particularity in the appended claims. My invention itself, however, together with further objects and advantages thereof, may best be understood by reference to the following description taken in connection with the accompanying drawing, in which Fig. 1 diagrammatically represents one embodiment of my invention; Figs. 2, 3, and 4am curves representative of. the operating characteristics of my invention; Figs. 5 and 6 are circuit diagrams of slightly different modifications of another form of my invention; and Fig. '7 is a circuit diagram of still another embodiment of my invention.

Referring first to Fig. 1, there is represented a detector and amplifier circuit. It may, for example, comprise the second detector of a superheterodyne radio receiver and the first stage of audio frequency amplification. As illustrated, the detector and amplifier elements are combined within a common tube envelope comprising a well known duplex-diode-triode. Of course, the elements may be in separate envelopes if desired.

High frequency carrier waves, modulated by desired audio signals, are impressed from any suitable signal receiving apparatus (not shown) upon the detector input through the tuned input transformer 10. The signal detector, which is of the conventional diode type, comprises the anode l2 and the common cathode l3.- The signal detector circuit extends from the anode l2 through the tuned secondary of the input transformer Ill, the diode load resistor l4 and diode condenser 15 in parallel, to the cathode l3.

The operation of this circuit is well known to those skilled in the art and need not be detailed here. The detector is elfective to demodulate the carrier waves impressed upon the transformer II]. In a superheterodyne receiver these carrier waves will generally be of an intermediate high frequency lower than the carrier frequency of the received signal, as is well known. The demodulation products developed across the diode load resistor it include audio frequency components and a direct current component. The audio frequency components are coupled to the grid it of the triode amplifier section of the tube ll through the usual audio coupling network connected across the diode loadresistor [4. As shown this comprises a blocking capacitor [1, a potentiometer l8-and-a suitable source of negative bias potential IQ for the gridllfih The audio frequency potentials impressed upon the grid l6 produce amplified currents in the audio amplifier output circuit which extends from the anode 21] through the coupling impedance 2|, and a suitable source of anode potential 22 to the common cathode I3. The amplified signal voltages appearing across the amplifier output may be coupled to any suitable utilization circuit (not shown) through the blocking capacitor'23. This circuit may of course comprise a further stage I of audio amplification or a sound translating device;

As is well known, interfering electrical disturbances of short duration and considerable magnitude are often received on the signal channel along with the desired signals. These also modulate the carrier waves in a random manner and seriously distort the desired signal, or even mask it completely, whenever they approach or exceed the magnitude of. the useful signal.

causes. For example, natural atmospheric static surges, man-made electrical disturbances, as

from high frequency apparatus, ignition systems and the like, and other sharp impulses fall within this category. The characteristics of these undesired noise impulses may vary widely, depending upon their source, but in all cases they have a deleterious effect upon the fidelity of signal reproduction, either by direct interference with the signal or through shock excitation effects in the receiving system.

In order to limit the effect of such noise impulses, a noise limiting network is provided in accordance with my invention. As shown in Fig. 1 this network includes a capacitor 24 in series with a resistor 25 connected across the output of the audio amplifier triode. Connected in shunt to the resistor 25 is a second diode detector comprising the anode 26 and the common cathode l3. This network functions substantially to reduce the interfering efiects of the noise impulses in a manner now to be described.

The desired audio frequency signals will generally extend over a range from approximately 15 or 20 cycles per second, as a lower limit, to about 4,000 to 10,000 cycles per second as an upper limit. The time constant of the resistancecapacitance network 24, 25 i chosen longer than the period of the lowest desired frequency within the audio signal range. That is, the time constant, which is equal to the product of resistance times capacity, should be longer than about th or th of a second. This range of frequencies from about 2 to or cycles per second is often designated as the syllabic frequency range, since such frequencies approximate those present in the syllabic inflections of the human voice. Expressed another way, these lower frequencies are the principal components present in the audio frequency envelope. 7

Referring now to Fig. 2, a train of audio frequency waves is graphically represented such as might be produced across the diode load resistor I 4 by a single syllable of human speech. Through the well-known action of the signal detection circuit, an audio frequency component is developed across the resistor l4 as indicated by the solid line curve 30. This audio frequency component may vary both in frequency and in amplitude as shown. The amplitude variations, which correspond to syllabic inflection in the illustrated example, are represented by the dotted envelope curve 3|. The demodulation products developed across the resistor I 4 also include a direct current component corresponding to the amplitude of the unmodulated carrier as indicated by the displacement of the alternating current axis ab below the zero voltage line by the distance 0a. The audio signal is shown below the zero axis, since the grounded end of resistor I4 is considered to be the zero reference point in the conventional manner. The upper terminal of resistor I4 therefore becomes negative with current flow in the detector circuit.

Assume now that an undesired noise impulse of greater magnitude than the desired signal and of short duration is impressed upon the input transformer 10. This might produce a single pair of alternate voltage peaks on the resistor I4, illustrated by 32 and 33, for example. The peak 32, which is in a direction to cause the detector diode to. become non-conductive, will be limited substantially to the zero axis by thehigh impedance of the detector diode to current flow from cathode to anode. However, the alternate peak 33 produces a sudden voltage rise across the diode load resistor l 4.

The distorted audio signal applied to the grid [6 and amplified by the triode would cause the plate voltage across the output coupling impedance 2| to vary somewhat as indicated in Fig. 3, in the absence of the limiter network 24, 25 and 2B. This curve i complementary to the curve of Fig. 2 and corresponding elements have been indicated by corresponding reference numerals with a prime affixed. Thus, it is seen that while the peak 32' is of small magnitude, the peak 33' represents a serious distortion. In actual operation under severe static conditions, the magnitude of the disturbing noise impulses may be many thousand times the signal level and may cause much more serious distortion of the signal envelope than is shown in the illustrated curve.

The noise limiting network 24, 25 and 26 functions as a peak detector of the audio signal in a manner similar to that of the signal diode detector circuit previously described. The main difference is in the longer time constant of the network 24, 25, whereby the detected voltage appearing across the resistor 25 corresponds substantially to the envelope of the audio frequency signals. This is represented graphically in Fig. 4 wherein the curve represents the audio voltage applied between the anode 26 and cathode l3 of the noise diode. The average voltage existing across the capacitor 24 is illustrated by the curve 34 and substantially follows the syllabic inflections of the audio envelope. Thus, through peak detection of the audio signal in the noise diode the anode 26 thereof is maintained nega tive with respect to the cathode l3 substantially in accordance with the curve 34. The noise impulse peak 33 greatly exceeds this negative bias and the noise diode is so poled that this peak renders the diode conductive. Thus a low impedance path for the noise impulse is provided between anode 20 and ground through th capacitor 24 and the diode 26l3. The Voltage produced across the output circuit by this impulse is consequently limited substantially to the zero voltage level as represented by the peak 33" in Fig. 4.

It will be observed that the capacitor l1 prevents the direct current component of the potential on resistor M from being applied to the triode and hence from appearing upon the noise diode. Thus, the noise limiting circuit responds only to the audio signal envelope and is independent of variations in the intensity of the received carrier wave, due to fading or the like.

Merely by way of illustration, and not in any sense by way of limitation, the following constants have been found to give satisfactory results in a particular application of the circuit of Fig. 1 to a superheterodyne radio receiver. The device II was a duplex-diode high mu triode, type GSQ'I. The resistor 25 was of 2.2 megohms resistance and the capacitor 24 of 0.1 mfd. capacity. Thus, the time constant of these elements was approximately th second, corresponding to a syllabic frequency of less than about 4 cycles which is well below the lowest audio frequency. By making the resistor 25 large the shunting effect of the noise limiting circuit upon audio frequency signals in the output of the audio amplifier was practically negligible. However, if a sharp noise pulse was applied to the circuit a low impedance path from anode 20 through capacitor 24 and diode 26|3 to ground was formed. Since the diode internal impedance was very low under these conditions compared to the internal anode to cathode resistance of the audio amplifier, the voltage impressed upon the output circuit due to the noise pulse was much reduced.

Fig. illustrates a modified form of my invention in which the noise limiting network is connected directly across the diode load resistor 44 of the signal detector circuit. High frequency modulated carrier waves impressed upon the input transformer 40 are demodulated by the diode detector 4|. The signal detection circuit is essentially the same as that of Fig. 1, with the exception that a filter comprising a, resistor 42 and capacitor 43 is included in the detector circuit to prevent carrier frequencies appearing across the diode load resistor 44 and diode capacitor 45. The audio frequency signals are taken off the resistor 46 through the blocking capacitor 65 and volume control potentiometer 4! in the usual manner. In this embodiment the noise limiting time constant network, comprising the resistor 48 and capacitor 49 serially interconnected with the load resistor 44, serves two functions. Since the time constant is chosen to be below the audio frequency range, as previously described, automatic volume control potentials may be taken from the upper terminal of the resistor 44 and applied through the resistor 48 and the conductor 50 to preceding tubes for controlling the amplification thereof in a well-known manner. Connection of the noise diode 5| across the resistor 48 completes the limiting network.

The operation of the noise limiting circuit of Fig. 5 is essentially the same as previously described in connection with the embodiment of Fig. 1. In this case the source of audio signals applied to the limiter network is the diode load resistance 44 rather than the anode circuit of the audio amplifier as in Fig. 1. The limiter network forms a shunt circuit to the load resistor 44. Since the diode 5| is conductive in the same direction as detector diode 4|, noise impulses exceeding the self bias developed by peak rectification of the audio signals in the diode 5| find a low impedance path to ground through diode 5| and capacitor 49. Thus the voltage across the load resistor 44, which is applied to the audio output through the capacitor 46 and potentiometer 41, is not appreciably increased by such impulses. It will again be noted that the direct current component appearing across the load resistor 44, and which may be utilized for automatic volume control through the conductor 50, cannot fiow through the capacitor 49. Thus the noise diode 5| receives no additional bias in response to variations in the input signal level but responds automatically to variations in the audio frequency level alone.

Fig. 6 represents a slight modification of the circuit of Fig. 5. Corresponding reference numerals have been given. The only difference lies in the fact that a separate filter network for automatic volume control circuit has been provided comprising the resistor 50 and capacitor 6|. This gives smoother automatic volume control potentials than the circuit of Fig. 5 in which the noise diode is shunted across the automatic volume control filter resistor 48. Voltage fluctuations across the capacitor 59 in Fig. 6 as a result of the rectifying action of the noise diode 5| are filtered from the automatic volume control circuit by means of the additional filter network 60, 6|.

In the embodiments of my invention illustrated in Figs. 1, 5 and 6 the noise limiting diode has been illustrated as of the thermionic type. It is to be understood that it may comprise any other suitable type of unilaterally conducting discharge device known to the art. For example, a copper oxide rectifier has been successfully employed in place of the diode 5| in an actual receiving system embodying the circuit of Fig. 5.

Fig. 7 represents a further embodiment of my invention. Modulated carrier waves are demodulated in the diode detector circuit 1|], which operates in the same manner as the detector circuits previously described. Audio frequency potentials appearing on the potentiometer 1| are amplified in a first audio amplifier 12, further amplified in the audio power amplifier l3 and finally impressed upon a suitable reproducing device, such as a loud speaker 14. In accordance with my invention the audio signals are applied to the grid 15 of the amplifier 13 through the resistance-capacitance coupling network 16 and 11. This network forms a grid leak and grid capacitor combination for developing a self bias for the grid of amplifier 13. The time constant of this network is syllabic so that it also performs the function of the noise limiter circuit of my invention in combination with the grid and cathode elements of the amplifier 13, which function as a diode rectifier. While the screen grid 18 may be connected directly to a positive source of potential, such as the battery 19, it is preferably connected thereto through a resistor 8!] for the purpose of limiting the anode current to a predetermined value, in a manner well known to the art. This is desirable since there is zero bias on the grid I5 in the absence of signals. An audio frequency by-pass capacitor 8| is also preferably connected between the screen grid and cathode to hold the screen grid at cathode potential as far as audio frequency signals are concerned in order to increase the conductance from control grid to anode.

Audio frequency signals impressed upon the amplifier 13 are rectified by the grid circuit and build up syllabic voltages across the capacitor 11 in the same manner as the noise limiter networks previously described. These low frequency voltages in no way interfere with the amplification of the audio signal. Now, if a sudden noise impulse is impressed on the grid 15, the grid to cathode impedance of the amplifier 13 drops to a very low value. This effectively places a very low load impedance across the amplifier tube 12, thereby reducing its amplification to a low level and effecting the limiting action.

While I have shown particular embodiments of my invention, it will of course be understood that I do not wish to be limited thereto since various modifications may be made, and I contemplate by the appended claims to cover any such modifications as fall within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. The combination, in a receiver of carrier waves modulated in accordance with audio frequency signals, of a detector for demodulating the received carrier waves, an amplifier for amplifying the audio frequency demodulation products of said detector, a resistance element and a capacity element serially connected in circuit across the output of said amplifier, said elements having a time constant approximating the syllabic inflections of the human voice, a unilaterally conducting path connected across said resistance element, and means comprising said path and said elements for producing across said path, by rectification of said products in said path itself, a syllabic bias which varies substantially in accordance with the envelope of said audio frequency signals, said path being poled to be conductive in response to voltage surges which are transmitted through said detector and said amplifier and which exceed said bias.

2. In a receiver of carrier waves modulated by desired frequencies extending over the audio frequency range and subject to interfering impulses, the combination of means for demodulating said waves, an impedance across which the audio frequency demodulation products of said waves are impressed, a resistance element and a capacity element serially connected with said impedance, said elements having a time constant longer than the period of the lowest desired frequency in said audio range and approximating the syllabic inflections of the human voice, a thermionic amplifier having a cathode, a grid and an anode, said cathode and grid being connected in circuit across said resistance element, said cathode and anode being connected in an audio output circuit,

and means responsive to the peak value of said demodulation products for impressing a grid bias on said amplifier substantially corresponding to the envelope of said desired frequencies, said means comprising said elements and utilizing grid rectification of said products in said cathode to grid circuit itself, whereby said grid circuit offers a low impedance conducting path to interfering impulses which exceed said envelope, thereby substantially to prevent said impulses from appearing in said output circuit.

ROBERT B. DOME. 

